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F.d. bemannade fyrplatser har INTE funnits i Mälaren.
Yta 1140 km2 (exklusive öarnas yta), längd 117 km, bredd 0,5-50 km, största djup 64 m.
Sjön uppvisar ingen större sammanhållande yta utan fördelas på en mängd fjärdar vikar och sund.
Mälaren utfaller i Östersjön vid Stockholm via Norrström och Söderström (Slussen).